It is sometimes desirable to locate the position of a station operable within a wireless, e.g., radio frequency (RF), network. For example, the United States Federal Communications Commission (FCC) has decreed that cellular telephone systems must implement systems to provide mobile telephone position information for use in emergency response, e.g., enhanced 911 (E911) emergency response. Additionally, the position of a station may be important for providing particular services, such as, for example, identifying subscribers and non-subscribers, resource allocation, network security, and location-sensitive content delivery, among other services.
In order to estimate a station's location, a system typically measures a metric that is a function of distance. A typical measured metric is signal strength, which decays logarithmically with distance in free space. Time information, such as time of arrival of a signal or time difference of arrival of a signal at diverse antennas, may be utilized as a measured metric from which distance information may be determined. Typically, several reference points are used with distance information derived from the measured metric in estimating location.
The use of global positioning system (GPS) receivers, which operate in conjunction with a network of middle earth orbit satellites orbiting the Earth to determine the receiver's position, has almost become ubiquitous in navigational applications. In such a GPS network, the aforementioned reference points are the satellites and the measured metric is the time of arrival of the satellite signal to the GPS receiver. The time of arrival of the satellite signal is typically directly proportional to the distance between the satellite and the GPS receiver due to a clear line of sight between the GPS receiver and satellite. By measuring the time of arrival associated with three satellites, a GPS receiver can calculate the longitude and latitude of the GPS receiver. By using time of arrival information with respect to a fourth satellite, a GPS receiver can also determine altitude.
In the aforementioned cellular networks, techniques including signal strength measurements and/or time difference of arrival have been implemented for location determination. For example, U.S. Pat. No. 6,195,556, the disclosure of which is incorporated herein by reference, teaches the use of signal strength measurements in combination with the time difference of arrival of a station's signal in determining the location of the station. Additionally, U.S. Pat. No. 6,195,556 teaches the use of mapping of received signal characteristics associated with particular positions (e.g., receive “signature” associated with each of a plurality of remote station locations) for use in determining a station's location. In the case of the aforementioned cellular network, the base transceiver stations (BTSs) are generally relied upon as the reference points from which distance determinations are made.
Wireless local area network (WLAN) location determination systems have been implemented in two phases: the offline phase and the online phase. In the offline phase, prediction or measurement of the fingerprint (e.g., signal strength, multipath characteristics, etcetera) of wireless access points at particular locations within the service area may be carried out. Location fingerprints may be predicted or measured off-line, such as when a network is being deployed, and are stored in a database resulting in a so-called radio map to relate the wireless signal information and coordinates of the known locations. In the online phase, the fingerprint associated with a remote station at an unknown location is measured during later operation of the network, and compared to the entries in the database. A location estimation algorithm is then applied to infer the location estimate for the unknown location. Location estimation algorithms include, for example but not limited to, triangulation, nearest neighborhood, K-nearest neighbor averaging, and history-based shortest path.
Previously, developing an accurate radio map for location determination required manual calibration throughout the network environment, meaning that before a location determination could be made, an engineer would actually have to physically go out and make calibration measurements at some specified points over the area covered by the network. Based on the manual measurements, the system would construct the radio map, and then make a location determination. This is known as supervised calibration or supervised training. Making manual calibration measurements is expensive and consumes significant manpower. Furthermore, because the wireless environment is constantly changing, the measured parameters are also changing, and repeating calibration to update the measurements is impractical and inefficient. Supervised training, requiring manual calibration, provides relatively accurate resolution, but over time, the accuracy fails as the networks parameters change. It is, therefore, desirable to eliminate the need for making costly and time consuming manual measurements.